It's because you are using the Meteotriviglio .gif icon set -- it hasn't been updated with all the new NWS image names.
Looking at the view-source of your wxforecast.php shows
<!-- point forecast processing -->
<!-- missing icon './forecast/images/nfzra_sn100.gif' .. using './forecast/images/na.gif' instead -->
<!-- missing icon './forecast/images/nip90.gif' .. using './forecast/images/na.gif' instead -->
So your icon set is missing the night-freezing-rain-snow100% and night-ice-pellets-90% icons.
You might try copying ./forecast/images/nrasn*.gif to nfzra_sn*.gif and likewise copy ip*.gif to nip*.gif
Or.. switch to the .jpg icons (which are complete).
For reference, here are the missing icon names I found back in March, 2017
cold10.gif
cold100.gif
cold20.gif
cold30.gif
cold40.gif
cold50.gif
cold60.gif
cold70.gif
cold80.gif
cold90.gif
fc.gif
fzra_sn.gif
fzra_sn10.gif
fzra_sn100.gif
fzra_sn20.gif
fzra_sn30.gif
fzra_sn40.gif
fzra_sn50.gif
fzra_sn60.gif
fzra_sn70.gif
fzra_sn80.gif
fzra_sn90.gif
fzrara10.gif
fzrara100.gif
fzrara20.gif
fzrara30.gif
fzrara40.gif
fzrara50.gif
fzrara60.gif
fzrara70.gif
fzrara80.gif
fzrara90.gif
hi_bkn10.gif
hi_bkn100.gif
hi_bkn20.gif
hi_bkn30.gif
hi_bkn40.gif
hi_bkn50.gif
hi_bkn60.gif
hi_bkn70.gif
hi_bkn80.gif
hi_bkn90.gif
hi_nbkn10.gif
hi_nbkn100.gif
hi_nbkn20.gif
hi_nbkn30.gif
hi_nbkn40.gif
hi_nbkn50.gif
hi_nbkn60.gif
hi_nbkn70.gif
hi_nbkn80.gif
hi_nbkn90.gif
hur_warn.gif
hur_watch.gif
hurr-noh10.gif
hurr-noh100.gif
hurr-noh20.gif
hurr-noh30.gif
hurr-noh40.gif
hurr-noh50.gif
hurr-noh60.gif
hurr-noh70.gif
hurr-noh80.gif
hurr-noh90.gif
hurr10.gif
hurr100.gif
hurr20.gif
hurr30.gif
hurr40.gif
hurr50.gif
hurr60.gif
hurr70.gif
hurr80.gif
hurr90.gif
hz.gif
minus_ra.gif
minus_ra10.gif
minus_ra100.gif
minus_ra20.gif
minus_ra30.gif
minus_ra40.gif
minus_ra50.gif
minus_ra60.gif
minus_ra70.gif
minus_ra80.gif
minus_ra90.gif
nblizzard.gif
nblizzard10.gif
nblizzard100.gif
nblizzard20.gif
nblizzard30.gif
nblizzard40.gif
nblizzard50.gif
nblizzard60.gif
nblizzard70.gif
nblizzard80.gif
nblizzard90.gif
ncold.gif
ncold10.gif
ncold100.gif
ncold20.gif
ncold30.gif
ncold40.gif
ncold50.gif
ncold60.gif
ncold70.gif
ncold80.gif
ncold90.gif
ndu.gif
nfc.gif
nfu.gif
nfzra.gif
nfzra10.gif
nfzra100.gif
nfzra20.gif
nfzra30.gif
nfzra40.gif
nfzra50.gif
nfzra60.gif
nfzra70.gif
nfzra80.gif
nfzra90.gif
nfzra_sn.gif
nfzra_sn10.gif
nfzra_sn100.gif
nfzra_sn20.gif
nfzra_sn30.gif
nfzra_sn40.gif
nfzra_sn50.gif
nfzra_sn60.gif
nfzra_sn70.gif
nfzra_sn80.gif
nfzra_sn90.gif
nip.gif
nip10.gif
nip100.gif
nip20.gif
nip30.gif
nip40.gif
nip50.gif
nip60.gif
nip70.gif
nip80.gif
nip90.gif
nmix10.gif
nmix100.gif
nmix20.gif
nmix30.gif
nmix40.gif
nmix50.gif
nmix60.gif
nmix70.gif
nmix80.gif
nmix90.gif
nra_fzra.gif
nra_fzra10.gif
nra_fzra100.gif
nra_fzra20.gif
nra_fzra30.gif
nra_fzra40.gif
nra_fzra50.gif
nra_fzra60.gif
nra_fzra70.gif
nra_fzra80.gif
nra_fzra90.gif
nra_sn.gif
nra_sn10.gif
nra_sn100.gif
nra_sn20.gif
nra_sn30.gif
nra_sn40.gif
nra_sn50.gif
nra_sn60.gif
nra_sn70.gif
nra_sn80.gif
nra_sn90.gif
nsn_ip.gif
nsn_ip10.gif
nsn_ip100.gif
nsn_ip20.gif
nsn_ip30.gif
nsn_ip40.gif
nsn_ip50.gif
nsn_ip60.gif
nsn_ip70.gif
nsn_ip80.gif
nsn_ip90.gif
nsnip.gif
nsnip10.gif
nsnip100.gif
nsnip20.gif
nsnip30.gif
nsnip40.gif
nsnip50.gif
nsnip60.gif
nsnip70.gif
nsnip80.gif
nsnip90.gif
ntor.gif
nwind_bkn.gif
nwind_few.gif
nwind_ovc.gif
nwind_sct.gif
nwind_skc.gif
pcloudy10.gif
pcloudy100.gif
pcloudy20.gif
pcloudy30.gif
pcloudy40.gif
pcloudy50.gif
pcloudy60.gif
pcloudy70.gif
pcloudy80.gif
pcloudy90.gif
pcloudyn10.gif
pcloudyn100.gif
pcloudyn20.gif
pcloudyn30.gif
pcloudyn40.gif
pcloudyn50.gif
pcloudyn60.gif
pcloudyn70.gif
pcloudyn80.gif
pcloudyn90.gif
ra_fzra.gif
ra_fzra10.gif
ra_fzra100.gif
ra_fzra20.gif
ra_fzra30.gif
ra_fzra40.gif
ra_fzra50.gif
ra_fzra60.gif
ra_fzra70.gif
ra_fzra80.gif
ra_fzra90.gif
ra_sn.gif
ra_sn10.gif
ra_sn100.gif
ra_sn20.gif
ra_sn30.gif
ra_sn40.gif
ra_sn50.gif
ra_sn60.gif
ra_sn70.gif
ra_sn80.gif
ra_sn90.gif
sn_ip.gif
sn_ip10.gif
sn_ip100.gif
sn_ip20.gif
sn_ip30.gif
sn_ip40.gif
sn_ip50.gif
sn_ip60.gif
sn_ip70.gif
sn_ip80.gif
sn_ip90.gif
snip.gif
snip10.gif
snip100.gif
snip20.gif
snip30.gif
snip40.gif
snip50.gif
snip60.gif
snip70.gif
snip80.gif
snip90.gif
tor.gif
tropstorm-noh10.gif
tropstorm-noh100.gif
tropstorm-noh20.gif
tropstorm-noh30.gif
tropstorm-noh40.gif
tropstorm-noh50.gif
tropstorm-noh60.gif
tropstorm-noh70.gif
tropstorm-noh80.gif
tropstorm-noh90.gif
tropstorm10.gif
tropstorm100.gif
tropstorm20.gif
tropstorm30.gif
tropstorm40.gif
tropstorm50.gif
tropstorm60.gif
tropstorm70.gif
tropstorm80.gif
tropstorm90.gif
ts_hur_flags.gif
ts_hur_flags10.gif
ts_hur_flags100.gif
ts_hur_flags20.gif
ts_hur_flags30.gif
ts_hur_flags40.gif
ts_hur_flags50.gif
ts_hur_flags60.gif
ts_hur_flags70.gif
ts_hur_flags80.gif
ts_hur_flags90.gif
ts_no_flag.gif
ts_no_flag10.gif
ts_no_flag100.gif
ts_no_flag20.gif
ts_no_flag30.gif
ts_no_flag40.gif
ts_no_flag50.gif
ts_no_flag60.gif
ts_no_flag70.gif
ts_no_flag80.gif
ts_no_flag90.gif
ts_nowarn.gif
ts_warn.gif
ts_watch.gif
wind_bkn.gif
wind_few.gif
wind_ovc.gif
wind_sct.gif
wind_skc.gif
